In the course of the processing, in particular the printing, of sheets it is frequently necessary to turn the sheet so that it be processed on both sides. For example, in known printing machines, a sheet turning device is provided, the device turning a sheet with the use of turning pockets. In such a sheet turning device comprising turning pockets, a sheet is first supplied in one direction by means of transport rollers and then fed into a sheet guide that is configured as a turning pocket. The leading edge of the sheet moves into the turning pocket, then the entire sheet is received by the turning pocket and subsequently moved out of the turning pocket with the formerly trailing edge now being the new leading edge. However, this widely used solution has the disadvantage that the turned sheet loses the allocation of the leading edge. The reason being that, in the turning pocket, the edge being located at the rear before the first turning operation now has become the leading element after the sheet has moved out of the turning pocket. This can result in inaccuracies in the printed image and in view of registration.
Furthermore, it is known to use an arrangement of several transport belts that are twisted relative to each other for turning sheets. In such a turning device, the transport directions are twisted together by 180° , and the sheets are turned by such a sheet turning unit for duplex printing. Such a sheet turning unit comprises four communicating deflecting rollers about which one transport belt, respectively, is being moved. The four deflecting rollers are arranged opposite each other on both sides of a transport path. The transport belts are guided around the deflecting rollers in such a manner that one transport belt is moved around a deflecting roller on the one side of the transport path, and the other transport roller is placed around a deflecting roller on the other side of the transport path. Then the transport rollers extend in a twisted manner over a swivel region in the center between the four transport rollers in such a manner that the transport belts are placed around the respective deflecting roller on the other side of the transport path. Due to this twisting or crossing of the transport belts, a sheet held between the transport belts is turned by 180° . One disadvantage of such sheet turning units is that, depending on the properties of the sheet, relative motions between the transport belt and the sheet may occur. Consequently, positioning inaccuracies may occur after the turning operation.
In order to provide offset functionality it is known in printing machines to transport a sheet in two directions, i.e., first in longitudinal direction and then in transverse direction of the sheet, with the use of two separately driven transport rollers in the sheet path. Thus the sheet is turned twice in transverse direction on an S-shaped curved path in order to ultimately be again aligned parallel to the original path. In doing so, the sheet can be transported further with a transverse offset. Due to the double curved movement of the sheets the transport rollers must have a very slim design because relative motions occur between the sheet and the driving transport rollers. With a wider configuration of the transport rollers the sheet could be damaged because the speed of the sheet is not uniform across the width of the transport roller. The reason being that the sheet moves more slowly at a point of contact with the transport roller closer to the inside of the curve than at a point of contact closer to the outside of the curve. Inasmuch as the transport rollers are very slim, counter rollers located opposite the transport wheels are subject to greater wear, and grooves may form at the pressure point of the transport rollers. In addition, an expensive software program and measuring system are required to enable subsequent corrections of the sheet alignment.
Furthermore, it is known to use diagonally moving transport belts with oppositely supported balls as the pressure points. As a result of the fact that diagonally extending transport belts are used the sheets must be aligned along a lateral abutment, however. This is a problem with thin sheets, in particular, because they tend to buckle. Also, the edges of the sheets may be damaged.